Daily Halacha with Rabbi Gidon Ben Moshe
(Rabbi Gidon Ben Moshe, one of the foremost contemporary Poskim in Israel, heads a Kollel devoted to the study of Hoshen Mishpat.)
The Hachamim instituted adding the special passage of "Reíseh" in the Birkat Hamazon of Shabbat. If one forgot to insert "Reíseh" after the first two meals of Shabbat, he must repeat the Birkat Hamazon. Since eating bread is mandatory in these two meals, the Birkat Hamazon requires "Reíseh." With regard to Seudah Shlishit, Maran rules that if one forgot to add "Reíseh," he does not repeat Birkat Hamazon, if he already completed it.
Some mistakenly derive from this Halacha that eating bread for Seudah Shlishit is not mandatory. They reason that Maranís ruling not to repeat Birkat Hamazon indicates that bread is not mandatory.
However, this is not true. Maran himself, in the Halachot of Seudah Shlishit, rules that Seudah Shlishit requires bread. While he does bring other opinions that permit fulfilling Seudah Shlishit on other foods, they are only cited as "Yesh Omrim" (some say), whereas the first ruling that requires bread is brought as the "Stam" (unattributed opinion). In such instances, the general principle dictates that the Halacha is in accordance with the "Stam," and thus bread is mandatory for Seudah Shlishit.
The reason that one does not repeat Birkat Hamazon if he forgot "Reíseh," is because the other opinions, which do not require bread, generate a Safek Berachot-uncertainty in the Halacha of Berachot, which is resolved by being lenient and refraining from repeating Birkat Hamazon.
Again, the Halacha requires eating bread to fulfill the Misva of Seudah Shlishit. If someone says that he has no appetite to eat bread, but he wants to rely on the opinion that allows eating cake, he should be told, "If you have room for cake, you have room for bread. Eat bread!" Itís not that he is unable to eat bread; he just doesnít have the desire to do so. However, if he would understand how important eating bread at Seudah Shlishit is, he would have the desire.
The Zohar Hakadosh teaches that Rabbi Shimon Bar Yochai, on his final day in this world, praised himself before Hashem that he never once missed out on fulfilling the three Seudot of Shabbat. The fact that this is what he chose to highlight from his multitude of other spiritual accomplishments in his life, teach us how important Seudah Shlishit is.
One must eat bread to fulfill the Misva of Seudah Shlishit on Shabbat. If he forgot to add "Reíseh" in the Birkat Hamazon of Seudah Shlishit, he does not need to repeat the Birkat Hamazon.